Method for automatically adjusting the filter parameters of a digital equalizer and reproduction device for audio signals for implementing such a method

ABSTRACT

A method is proposed for automatically adjusting the filter parameters—center frequency, quality and amplification or attenuation—of at least one digital equalizer which is a component of a reproduction device for audio signals in a vehicle passenger compartment. To that end, first of all, the acoustical frequency response of the passenger compartment is ascertained. The inadequacies in the acoustics of the passenger compartment in the form of local maxima and minima in the measured frequency response are then determined. On this basis, the filter parameters are adjusted automatically so that at least a portion of these inadequacies is compensated. A reproduction device for audio signals for implementing this method is also proposed.

FIELD OF THE INVENTION

The present invention relates to a method for automatically adjustingthe filter parameters—center frequency, quality and amplification orattenuation—of at least one digital equalizer which is a component of areproduction device for audio signals in a vehicle passengercompartment. The invention also relates to a reproduction device foraudio signals for implementing such a method, having a loudspeakerdevice and having an audio processor which includes at least one digitalequalizer, is arranged in the signal path between at least one signalsource and the loudspeaker device, and is connected to a controlprocessor via a control bus.

BACKGROUND INFORMATION

Certain car radio devices, known from practice, are based on theso-called 2-IC technology. In these car radio devices, two or threefreely programmable audio filters are integrated into the signal path.These digital parametric equalizers (DPE) are available to the user tocompensate for acoustical shortcomings in the passenger compartment. Theuser is able to vary each filter with respect to center frequency,quality, i.e. filter width, and amplification or attenuation, in orderto compensate for excessive rises and so-called holes in the acousticalfrequency response of the passenger compartment.

However, this proves to be problematic in practice, since the user mustknow the acoustics of his/her vehicle very well to optimally adjust theequalizers, and it is very difficult to ascertain the acousticalfrequency response solely by listening, without metrological aid. Theoperating instructions of the known car radio devices are only able toprovide very limited assistance for the best possible adjustment of theequalizers, since on no account is it possible to consider all types ofvehicles here, and by no means the great number of individual layoutvariants, as well as loudspeaker and amplifier configurations.

Moreover, car radio devices are known having an audio module, integratedin the signal path, on which a graphic equalizer is implemented with theaid of a digital signal processor. The seven or nine bands of such agraphic equalizer are fixed in their center frequency and quality, andare only variable in their amplification. The separate audio module ofthese car radio devices permits an automatic calibration of the graphicequalizer. To that end, the acoustics in the passenger compartment aremeasured with the aid of a microphone connected to the audio module viaan analog-to-digital converter. Using a special software, the graphicequalizer is subsequently adjusted in such a way that the inadequaciesof the acoustics are compensated for in the best way possible.

The use of a graphic equalizer to compensate for the inadequacies in theacoustics of a passenger compartment proves to be problematic inpractice. As already mentioned, the center frequencies of the equalizerbands of a graphic equalizer are fixed. As a rule, they are spaced apartby a minimum of one octave in the case of nine bands. Thus, it is notpossible to optimally compensate for narrow resonance rises, which liebetween the equalizer bands, in the acoustical frequency response of thepassenger compartment. Moreover, the additional audio module having thedigital signal processor for implementing the graphic equalizer and forcalibrating this equalizer is relatively cost-intensive.

SUMMARY OF THE INVENTION

With the present invention, it is now proposed to adjust the filterparameters—center frequency, quality and amplification or attenuation—ofthe digital equalizer(s) automatically, in order to relieve the user ofthe difficult task of adapting the digital equalizer(s) to the specialacoustics of his/her vehicle passenger compartment.

This is achieved according to the present invention by a method forautomatically adjusting the filter parameters, in which first of all,the acoustical frequency response of the passenger compartment isascertained, then the shortcomings in the acoustics of the passengercompartment in the form of local maxima and minima in the frequencyresponse are determined, and thereupon the filter parameters areadjusted automatically so that at least a portion of these shortcomingsis compensated for.

Moreover, a reproduction device of the type indicated at the outset isproposed which, according to the present invention, to automaticallyadjust the digital equalizer(s), includes a noise generator, via which anoise signal may be supplied to the equalizer. In addition, the controlprocessor includes means, via which the filter parameters are adjustableso that the equalizer has a bandpass characteristic with a narrowbandwidth, the center frequency being variable over the audio spectrum.To capture the signal emitted by the loudspeaker device into thepassenger compartment and to determine the frequency response, at leastone microphone having evaluation means is provided. Finally, the controlprocessor also includes means via which the filter parameters areadjustable, taking into account the measured frequency response.

According to the present invention, it has become known that anautomatic adjustment of the filter parameters of the digital equalizersof a reproduction device for audio signals in a passenger compartment isuseful, since when optimizing the filter parameters, it is necessary toconsider the individual acoustical properties of the passengercompartment, arranged and equipped specific to the user, and theseproperties may be detected best using metrological means. By varying notonly the amplification and attenuation, respectively, of the equalizers,but also the center frequencies and qualities, it is possible tocompensate for the shortcomings in the acoustics of the passengercompartment very well, regardless of the position and the width of theexcessive rises and holes in the measured frequency response.

Furthermore, it has become known according to the present invention thatthe equalizers to be calibrated, because of their programmability, maybe used first of all for determining the acoustical frequency responseof the passenger compartment before the filter parameters are adjustedto compensate for the inadequacies in the measured frequency response.It has also become known that the filter parameters may be optimizedwith the aid of a suitable additional software of the control processor,present anyway, of the car radio device. Thus, all in all, no additionalaudio module having a digital signal processor is necessary within theframework of the present invention, but rather only a microphoneamplification and rectification circuit which is coupled to theanalog-to-digital converter present in the control processor. In thismanner, only a very small additional outlay for hardware and software,and therefore costs, is necessary for the automatic adjustment of thefilter parameters proposed in the present invention.

In principle, there are various possibilities for determining theacoustical frequency response of the vehicle passenger compartmentwithin the framework of the method according to the present invention.In one advantageous variant, the loudspeaker device of the reproductiondevice is triggered in succession by bandpass noise signals havingdifferent center frequencies. The frequency bands, set in each case inthe form of a bandpass noise signal, cover the entire audio spectrum.The frequency response to be determined is now ascertained in the formof frequency measuring points for the individual frequency bands. Thesound level of the signal which, in this case, is emitted by theloudspeaker device into the passenger compartment, may simply bedetermined as a frequency measuring point for a specific frequency band.

In view of minimizing the hardware and software expenditure, it provesto be advantageous to generate the bandpass noise signals forascertaining the acoustical frequency response of the passengercompartment using the equalizer to be adjusted itself. Since both thecenter frequency and the quality of the equalizer are freelyprogrammable, the filter parameters may be adjusted so that a bandpasscharacteristic having a narrow bandwidth at a predefined centerfrequency results for the equalizer. From a noise signal supplied to it,the equalizer then generates the desired bandpass noise signal or asuccession of bandpass noise signals which cover the entire audiospectrum.

In principle, there are also various possibilities within the frameworkof the method of the present invention for the automatic determinationand adjustment of the filter parameters. In one advantageous variant, aplurality of normalized equalizer curve patterns of different qualityare stored for this purpose. To determine the filter parameters, foreach curve pattern and each local maximum determined in the measuredfrequency response, the center frequency of the curve pattern is nowshifted to the local maximum, and an attenuation is determined byscaling the curve pattern to the level of this local maximum. The filtercorresponding to this scaled curve pattern is then used on the measuredfrequency response, and the deviation of the resulting frequencyresponse from a target frequency response is determined. In this way,for each potential center frequency of the equalizer, as many errorvalues for the deviation from the target frequency response aredetermined as there are curve patterns or qualities stored. The filterparameters—center frequency, attenuation and quality—of that curvepattern for which the smallest error value has been determined arefinally taken as the basis for the automatic adjustment of theequalizer.

In view of the different perception of resonances and holes in thefrequency response, as well as the general dependence of the perceptionon the frequency of the audio signal, it is advantageous to weight theindividual deviations when determining the deviation of a filteredfrequency response from the target frequency response. In so doing, itproves to be useful to weight positive individual deviations morestrongly than negative individual deviations, so that any remainingexcessive rises in the frequency response are evaluated as worse thanthe holes which are far more uncritical psychoacoustically.Alternatively or in addition thereto, psychoacoustically criticalfrequency ranges may be weighted more strongly than psychoacousticallyuncritical frequency ranges.

Moreover, it is advantageous if, when determining the deviation of afiltered frequency response from the target frequency response, thelevel of the local maximum or the resonance corresponding to it is takeninto account, so that narrow, high resonances lead to a smaller errorvalue compared to wider, less high resonances, and therefore arepreferably eliminated.

If the filter parameters of a plurality of digital equalizers must beadjusted automatically, it is advantageous to determine the filterparameters of the individual equalizers in succession, in that in eachcase, prior to determining the filter parameters of one equalizer, theequalizer(s) adjusted before are used on the measured frequencyresponse.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the block diagram of a reproduction device for audiosignals for implementing the method of the present invention.

DETAILED DESCRIPTION

Reproduction device 1 shown in the FIGURE is used for reproducing audiosignals in a vehicle passenger compartment; the audio signals may begenerated by different audio sources 2, 3, such as radio, CD, CC, etc.Reproduction device 1 includes a loudspeaker device 4 and an audioprocessor 5 that is arranged in the signal path between audio sources 2,3 and loudspeaker 4 and that has two freely adjustable digitalequalizers 6, 7, via which the signals from different audio sources 2, 3are fed to loudspeaker device 4. Of course, more than two equalizers mayalso be provided here. To adjust the filter parameters—center frequency,quality and amplification or attenuation—a control processor 8 sendssuitable filter parameters via a control bus 9 to audio processor 5.

To determine the frequency response of the passenger compartment,reproduction device 1 also includes a noise generator 10, via which anoise signal may be supplied to equalizers 6, 7. Noise generator 10 isimplemented here as additional software in audio processor 5, and, ifnecessary, may be started via control processor 8. Alternatively, thenoise signal could also be generated by an external noise source asadditional audio source, for example, with the aid of an appropriate CDor a suitably adjusted tuner.

Control processor 8 also includes means via which the filter parametersmay be adjusted in such a way that equalizers 6, 7 have a bandpasscharacteristic with a narrow bandwidth, i.e. with a quality on the orderof magnitude of 8, the center frequency being variable over the audiospectrum. In this way, with the aid of noise generator 10 and viaequalizers 6, 7, loudspeaker device 4 may be triggered by a bandpassnoise signal.

When the calibration of equalizers 6, 7 has been started, for example,by a keystroke, control processor 8 varies the filter parameters indefined time sequence, so that the center frequency of the bandpassfilter decreases, for example, in the one-third-octave interval from thehighest to the lowest frequency to be adjusted. The signals, which arethen emitted in each case via loudspeaker device 4 into the passengercompartment, are detected with the aid of a microphone 11 and evaluatedby suitable evaluation means 12 for determining the frequency responseof the passenger compartment. To that end, the signals sensed bymicrophone 11 are amplified in an operational amplifier circuit,subjected to a logarithmic procedure and rectified, so that a directvoltage is present at the output of this circuit. The magnitude of thisdirect voltage is proportional to the sound level or sound pressure inthe passenger compartment for the frequency band, which is adjusted bythe respective bandpass noise signal. The sound level for the entireaudio spectrum is detected by the tuning of equalizers 6, 7.

The direct voltage representing the sound level is sampled by ananalog-to-digital converter 13 of control processor 8, so that after thetuning of all frequencies or frequency bands to be measured with thecorresponding voltage values, a precise image of the acousticalfrequency response of the passenger compartment is available to controlprocessor 8. The absolute frequency response value or amplituderesponse, and not the phase response, is designated exclusively here asthe frequency response.

Control processor 8 now ascertains the inadequacies, i.e. the resonancesand holes, in the acoustics of the passenger compartment in the form oflocal maxima and minima in the measured frequency response, anddetermines the filter parameters—center frequency, amplification andquality—of equalizers 6, 7, so that these inadequacies are compensatedfor as well as possible.

The total additional expenditure compared to a car radio device whoseequalizers are not adjustable automatically is in an additional hardware10 or additional software for generating a noise signal, an additionalsoftware in control processor 8 which takes over the sequencing controlof the calibration process as well as the ascertainment of the bestfilter parameter setting, and an additional hardware 12 for theamplification, logarithmation and rectification of the microphonesignal.

To ascertain the best possible setting of the filter parameters,normalized equalizer curve patterns having different quality are storedin audio processor 5.

In one advantageous variant of the method according to the presentinvention, first of all the resonances, i.e. the local maxima, in thefrequency response, measured and adjusted by the frequency response ofthe microphone, are determined. For each curve pattern and each of theselocal maxima, the following work steps are then carried out:

-   -   The center frequency of the curve pattern is shifted to the        local maximum and scaled using the level of the resonance, i.e.        the level of the maximum. The frequency response resulting        therefrom is subtracted from the measured frequency response,        which corresponds to the use of a filter having the properties        of the shifted and scaled curve pattern on the measured        frequency response.    -   The deviation of the resulting frequency response from a        predefined target frequency response is then ascertained. As a        rule, the target frequency response is linear, but a raising or        lowering of certain frequency ranges may also be provided. The        deviation is ascertained by weighted summation of the amounts of        the individual deviations at the frequency points, and is a        measure for how good the equalization is for the individual        shifted and scaled curve patterns. The greater the value of the        deviation, the poorer the equalizing. Positive deviations are        weighted double compared to negative deviations, so that any        remaining excessive rises in the frequency response are        evaluated as worse than the psychoacoustically far less critical        holes. A different weighting of individual frequency ranges is        also conceivable here, since resonances in certain frequency        ranges are more critical than in others. The result of this        weighted summation corresponds in principle to the “area”        between the target curve and the real curve, the portion above        the target curve being evaluated double. An error value now        exists for each curve pattern, i.e. for each quality, and for        each local maximum in the measured frequency response.    -   The level of the respective resonance, i.e. of the corresponding        maximum, is also subtracted from this error value. Smaller error        values are thereby allocated to narrow high resonances, than to        wide, less high resonances having the same “error area”. The        former are thus preferably eliminated, which is useful from the        psychoacoustical standpoint.

For each potential equalizer center frequency, as many error values nowexist as there are curve patterns or qualities stored. Theparameters—amplification or scaling, center frequency and quality—of theshifted and scaled curve pattern for which the smallest error value hasbeen determined are now selected as filter parameters.

The frequency response determined in this way for the first equalizer isadded to the measured frequency response. These same work steps are thencarried out for ascertaining the filter parameters of the secondequalizer; here then, the measured frequency response of the passengercompartment is not taken as a basis, but rather the frequency responseof the passenger compartment filtered by the first equalizer.

1. A method for automatically adjusting at least one filter parameter ofat least one digital equalizer that is a component of a reproductiondevice for an audio signal in a vehicle passenger compartment,comprising: ascertaining an acoustical frequency response of the vehiclepassenger compartment; determining an inadequacy in an acoustics of thevehicle passenger compartment in the form of one of a local maxima and alocal minima in the acoustical frequency response; automaticallyadjusting the at least one filter parameter so that at least a portionof the inadequacy is compensated for; and storing a plurality ofnormalized curve patterns of different quality for the automaticadjustment of the at least one filter parameter, wherein: eachdetermined local maximum in the acoustical frequency response is apotential center frequency of the at least one digital equalizer; foreach determined local maximum in the acoustical frequency response: foreach of the normalized curve patterns: (a) the center frequency of therespective curve pattern is shifted to the respective local maximum, (b)an attenuation is determined by scaling the respective curve pattern tothe level of the respective local maximum, (c) the filter correspondingto the respective scaled curve pattern is used on the acousticalfrequency response to output a filtered frequency response, and (d) adeviation of the filtered frequency response from a target frequencyresponse is determined, the deviation representing a corresponding errorvalue; and the performance of (a)-(d) for each of the plurality of curvepatterns results in as many error values for the respective potentialcenter frequency as there are curve patterns stored; and those of the atleast one filter parameter of a particular one of the shifted and scaledcurve patterns that has led to a smallest one of the error values aretaken as the basis for the automatic adjustment of the at least onedigital equalizer.
 2. The method as recited in claim 1, wherein: the atleast one filter parameter includes a center frequency, a quality, andone of an amplification and an attenuation.
 3. The method as recited inclaim 1, wherein: the acoustical frequency response of the vehiclepassenger compartment is ascertained by: triggering a loudspeaker deviceof the reproduction device in succession by bandpass noise signalshaving different center frequencies, wherein frequency bands, adjustedin each case in the form of a bandpass noise signal, cover an entireaudio spectrum, and ascertaining the acoustical frequency response inthe form of frequency measuring points for individual ones of thefrequency bands, wherein a sound level of a signal emitted by theloudspeaker device into the vehicle passenger compartment is determinedas the frequency measuring point for a specific frequency band.
 4. Themethod as recited in claim 3, further comprising: generating thebandpass noise signal by operating the at least one digital equalizer,in that a noise signal is supplied to the at least one digitalequalizer, and the at least one filter parameter is adjusted so that abandpass characteristic having a narrow bandwidth at a predefined centerfrequency results for the at least one digital equalizer.
 5. The methodas recited in claim 1, further comprising: for each deviationdetermination, weighting, for a plurality of frequency points of therespective filtered frequency response, respective individualcorresponding deviations from the target frequency response.
 6. Themethod as recited in claim 5, wherein: a positive individual deviationis weighted more strongly than a negative individual deviation.
 7. Themethod as recited in claim 5, wherein: some frequency ranges areweighted more strongly than other frequency ranges.
 8. The method asrecited in claim 1, wherein: a level of one of the local maximum and aresonance corresponding to the level of the local maximum is taken intoaccount when determining the deviation of a filtered frequency responsefrom the target frequency response, so that resonances which are narrowand high compared to wider, less high resonances are eliminated.
 9. Themethod as recited in claim 1, wherein: the at least one digitalequalizer includes plural digital equalizers, the at least one filterparameter includes plural filter parameters, the filter parameters of atleast two digital equalizers are adjusted automatically, and the filterparameters are determined in succession, in that, in each case, prior todetermining the filter parameters of one of the digital equalizer, atleast one of the equalizer adjusted before are used on the acousticalfrequency response.
 10. A reproduction device for an audio signal,comprising: a loudspeaker device; an audio processor that includes atleast one digital equalizer; a control bus; at least one microphoneincluding an evaluation device for detecting a signal emitted by theloudspeaker device into a vehicle passenger compartment and fordetermining a frequency response; a control processor connected to theaudio processor via the a control bus, the control processor includingan arrangement for determining an inadequacy in an acoustics of thevehicle passenger compartment and for automatically adjusting at leastone filter parameter to compensate at least a portion of the inadequacy,the adjustment taking into account the frequency response; wherein: eachdetermined local maximum of the frequency response is a potential centerfrequency of the at least one digital equalizer; for each determinedlocal maximum of the frequency response: the arrangement is configuredto, for each of a plurality of stored normalized curve patterns ofdifferent quality for the adjustment of the at least one filterparameter: (a) shift the center frequency of the respective curvepattern to the respective local maximum; (b) determine an attenuation byscaling the respective curve pattern to the level of the respectivelocal maximum; (c) use the filter corresponding to the respective scaledcurve pattern on the frequency response to output a filtered frequencyresponse; and (d) determine a deviation of the filtered frequencyresponse from a target frequency response, the deviation representing acorresponding error value; and the performance for the respectivefrequency response of (a)-(d) for each of the plurality of curvepatterns results in as many error values for the respective potentialcenter frequency as there are curve patterns stored; and the arrangementis configured to take those of the at least one filter parameter of oneof the shifted and scaled curve patterns that has led to a smallest oneof the error values as the basis for the automatic adjustment of the atleast one digital equalizer.
 11. The reproduction device as recited inclaim 10, further comprising: at least one signal source; and a noisegenerator via which a noise signal can be supplied to the at least oneequalizer; wherein: the audio processor is arranged in a signal pathbetween the at least one signal source and the loudspeaker device; thearrangement is configured to adjust the at least one filter parameter sothat at least one digital equalizer has a bandpass characteristic with anarrow bandwidth; and a center frequency of the at least one equalizeris variable over an audio spectrum.
 12. The reproduction device asrecited in claim 11, wherein: the noise generator is implemented in theaudio processor.
 13. The reproduction device as recited in claim 11,wherein: the noise generator includes an additional external signalsource.
 14. The reproduction device as recited in claim 10, wherein: theevaluation device includes an arrangement for performing anamplification, a logarithmation, and a rectification of the audiosignal.